This invention pertains to a process and catalyst for the direct oxidation of olefins, such as propylene, by oxygen to olefin oxides, such as propylene oxide.
Olefin oxides, such as propylene oxide, are used to alkoxylate alcohols to form polyether polyols, such as polypropylene polyether polyols, which find significant utility in the manufacture of polyurethanes and synthetic elastomers. Olefin oxides are also important intermediates in the manufacture of alkylene glycols, such as propylene glycol and dipropylene glycol, and alkanolamines, such as isopropanolamine, which are useful as solvents and surfactants.
Propylene oxide is produced commercially via the well-known chlorohydrin process wherein propylene is reacted with an aqueous solution of chlorine to produce a mixture of propylene chlorohydrins. The chlorohydrins are dehydrochlorinated with an excess of alkali to produce propylene oxide. This process suffers from the production of a low concentration salt stream. (See K. Weissermel and H. J. Arpe, Industrial Organic Chemistry, 2nd ed., VCH Publishers, Inc., New York, N.Y., 1993, p. 264-265.)
Another well-known route to olefin oxides relies on the transfer of an oxygen atom from an organic hydroperoxide or peroxycarboxylic acid to an olefin. In the first step of this oxidation route, a peroxide generator, such as isobutane or acetaldehyde, is autoxidized with oxygen to form a peroxy compound, such as t-butyl hydroperoxide or peracetic acid. This compound is used to epoxidize the olefin, typically in the presence of a transition metal catalyst, including titanium, vanadium, molybdenum, and other heavy metal compounds or complexes. Along with the olefin oxide produced, this process disadvantageously produces equimolar amounts of a coproduct, for example an alcohol, such as t-butanol, or an acid, such as acetic acid, whose value must be captured in the market place. (See Industrial Organic Chemistry, ibid., p. 265-269.)
Although the direct oxidation of ethylene by molecular oxygen to ethylene oxide has been commercialized with a silver catalyst, it is known that the analogous direct oxidation of propylene exhibits a low selectivity to the olefin oxide. Disadvantageously large amounts of acrolein and oxygen-containing C1-3 byproducts are produced, as taught in Industrial Organic Chemistry, ibid., p. 264. Some patents represented by U.S. Pat. Nos. 4,007,135 and 4,845,253, teach the use of metal-promoted silver catalysts for the oxidation of propylene with oxygen to propylene oxide. Among the metal promoters disclosed are gold, beryllium, magnesium, calcium, barium, strontium, and the rare earth lanthanides. These promoted silver catalysts also exhibit low selectivities to the olefin oxide.
Alternatively, EP-A1-0,709,360 discloses a process of oxidizing an unsaturated hydrocarbon, such as propylene, with oxygen in the presence of hydrogen and a catalyst to form an epoxide, such as propylene oxide. Gold deposited on titanium dioxide, further immobilized on a carrier such as silica or alumina, is taught as the catalyst composition. The catalyst exhibits lower olefin oxide selectivity and less efficient hydrogen consumption when operated at higher temperatures. Additionally, the catalyst has a short run time.
PCT publication WO-A1-96/02323 discloses the oxidation of an olefin, including propylene, with oxygen in the presence of hydrogen and a catalyst to form an olefin oxide. The catalyst is a titanium or vanadium silicalite containing at least one platinum group metal, and optionally, an additional metal selected from gold, iron, cobalt, nickel, rhenium, and silver. The productivity of olefin oxide is low in this process.
In view of the above, a need continues to exist in the chemical industry for an efficient direct route to propylene oxide and higher olefin oxides from the reaction of oxygen with C3 and higher olefins. The discovery of such a process which simultaneously achieves high selectivity to the olefin oxide at an economically advantageous conversion of the olefin would represent a significant achievement over the prior art. For commercial viability such a process would also require that the catalyst exhibit a long lifetime.
U.S. Pat. Nos. 4,839,327 and 4,937,219 represent additional art disclosing a composition comprising gold particles having a particle size smaller than about 500 xc3x85 immobilized on an alkaline earth oxide or titanium dioxide or a composite oxide of titanium dioxide with an alkaline earth oxide. A preparation of this composition involves deposition of a gold compound onto the alkaline earth oxide, titanium dioxide, or the composite oxide, followed by calcination so as to produce metallic gold of a particle size smaller than about 500 xc3x85. This teaching is silent with respect to depositing the gold particles on a titanosilicate and to a process for producing olefin oxides.
This invention is a novel process of preparing an olefin oxide directly from an olefin and oxygen. The process comprises contacting an olefin having at lcast three carbon atoms with oxygen in the presence of hydrogen and in the presence of a catalyst under process conditions sufficient to produce the corresponding olefin oxide. The unique catalyst which is employed in the process of this invention comprises gold on a titanosilicate.
The novel process of this invention is useful for producing an olefin oxide directly from oxygen and an olefin having three or more carbon atoms. Unexpectedly, the process of this invention produces the olefin oxide in a remarkably high selectivity. Partial and complete combustion products, such as acrolein and carbon dioxide, which are found in large amounts in many prior art processes, are produced in lesser amounts in the process of this invention. Significantly, the process of this invention can be operated at a high temperature, specifically greater than about 120xc2x0 C., while maintaining a high selectivity to olefin oxide. Operation at higher temperatures advantageously provides steam credits from the heat produced. Accordingly, the process of this invention can be integrated into a total plant design wherein the heat derived from the steam is used to drive additional processes, for example, the separation of the olefin oxide from water. Even more advantageously, since water is produced as a byproduct of this process, the hydrogen efficiency, as measured by the water to olefin oxide molar ratio, is good. Most advantageously, the process in its preferred embodiments exhibits an olefin conversion which is good.
In another aspect, this invention is a unique catalyst composition comprising gold on a titanosilicate.
The novel composition of this invention can be effectively used in the aforementioned direct oxidation of an olefin having three or more carbon atoms to the corresponding olefin oxide. Besides being active and highly selective for the olefin oxide, the catalyst exhibits evidence of a long lifetime. As a further advantage, when partially or completely spent, the catalyst is easy to regenerate. Accordingly, this unique catalyst exhibits highly desirable properties for the process of oxidizing propylene and higher olefins to their corresponding olefin oxides.
The novel process of this invention comprises contacting an olefin having at least three carbon atoms with oxygen in the presence of hydrogen and an epoxidation catalyst under process conditions sufficient to prepare the corresponding olefin oxide. In one preferred embodiment, a diluent is employed with one or more of the reactants, as described in detail hereinafter. The relative molar quantities of olefin, oxygen, hydrogen, and optional diluent can be any which are sufficient to prepare the desired olefin oxide. In a preferred embodiment of this invention, the olefin employed is a C3-12 olefin, and it is converted to the corresponding C3-12 olefin oxide. In a more preferred embodiment, the olefin is a C3-8 olefin, and it is converted to the corresponding C3-8 olefin oxide. In a most preferred embodiment, the olefin is propylene, and the olefin oxide is propylene oxide.
The novel catalyst which is employed in the epoxidation process of this invention comprises gold on a titanosilicate. The titanosilicate is generally characterized as having a framework structure formed from SiO44xe2x88x92 tetrahedra wherein a portion of the silicon atoms is replaced with titanium atoms. Preferably, the titanosilicate is a porous titanosilicate. In this preferred form, a series of pores or channels or cavities exists within the framework structure, thereby giving the titanosilicate its porous properties. A most preferred form of the titanosilicate is titanium silicalite-1 (TS-1) having a crystalline structure, as determined by X-ray diffraction (XRD), which is isomorphous to the structure of zeolite ZSM-5 and the pure silica form of ZSM-5 known as xe2x80x9csilicalitexe2x80x9d. In a more preferred embodiment of the catalyst, the gold exists in the form of clusters having an average particle size of about 10 xc3x85 or greater, as determined by transmission electron microscopy (TEM).
Any olefin containing three or more carbon atoms can be employed in the process of this invention. Monoolefins are preferred, but compounds containing two or more olefins, such as dienes, can also be employed. The olefin can be a simple hydrocarbon containing only carbon and hydrogen atoms. Alternatively, the olefin can be substituted at any of the carbon atoms with an inert substituent. The term xe2x80x9cinertxe2x80x9d, as used herein, requires the substituent to be substantially non-reactive in the process of this invention. Suitable inert substituents include, but are not limited to, halide, ether, ester, alcohol, and aromatic moieties, preferably, chloro, C1-12 ether, ester, and alcohol moieties, and C6-12 aromatic moieties. Non-limiting examples of olefins which are suitable for the process of this invention include propylene, 1-butene, 2-butene, 2-methylpropene, 1-pentene, 2-pentene, 2-methyl-1-butene, 2-methyl-2-butene, 1-hexene, 2-hexene, 3-hexene, and analogously, the various isomers of methylpentene, ethylbutene, heptene, methylhexene, ethylpentene, propylbutene, the octenes, including preferably 1-octene, and other higher analogues of these; as well as butadiene, cyclopentadiene, dicyclopentadiene, styrene, a-methylstyrene, divinylbenzene, allyl chloride, allyl alcohol, allyl ether, allyl ethyl ether, allyl butyrate, allyl acetate, allyl benzene, allyl phenyl ether, ally propyl ether, and allyl anisole. Preferably, the olefin is an unsubstituted or substituted C3-12 olefin, more preferably, an unsubstituted or substituted C3-8 olefin. Most preferably, the olefin is propylene. Many of the aforementioned olefins are available commercially; others can be prepared by chemical processes known to those skilled in the art.
The quantity of olefin employed in the process can vary over a wide range provided that the corresponding olefin oxide is produced. Generally, the quantity of olefin employed depends upon the specific process features, including for example, the design of the reactor, the specific olefin, and economic and safety considerations. Those skilled in the art can determine a suitable range of olefin concentrations for the specific process features desired. Generally, on a molar basis an excess of olefin is used relative to the oxygen, because this condition enhances the productivity to olefin oxide. Based on the process conditions disclosed herein, typically, the quantity of olefin is greater than about 1, preferably, greater than about 10, and more preferably, greater than about 20 mole percent, based on the total moles of olefin, oxygen, hydrogen, and optional diluent. Typically, the quantity of olefin is less than about 99, preferably, less than about 85, and more preferably, less than about 70 mole percent, based on the total moles of olefin, oxygen, hydrogen, and optional diluent.
Oxygen is also required for the process of this invention. Any source of oxygen is acceptable, including air and essentially pure molecular oxygen. Other sources of oxygen may be suitable, including ozone, and nitrogen oxides, such as nitrous oxide. Molecular oxygen is preferred. The quantity of oxygen employed can vary over a wide range provided that the quantity is sufficient for producing the desired olefin oxide. Ordinarily, the number of moles of oxygen per mole of olefin used in the feedstream is less than 1. Under these conditions the conversion of olefin and selectivity to olefin oxide are enhanced while the selectivity to combustion products, such as carbon dioxide, is minimized. Preferably, the quantity of oxygen is greater than about 0.01, more preferably, greater than about 1, and most preferably greater than about 5 mole percent, based on the total moles of olefin, hydrogen, oxygen, and optional diluent. Preferably, the quantity of oxygen is less than about 30, more preferably, less than about 25, and most preferably less than about 20 mole percent, based on the total moles of olefin, hydrogen, oxygen, and optional diluent. Above about 20 mole percent, the concentration of oxygen may fall within the flammable range for olefin-hydrogen-oxygen mixtures.
Hydrogen is also required for the process of this invention. In the absence of hydrogen, the activity of the catalyst is significantly decreased. Any source of hydrogen can be used in the process of this invention, including for example, molecular hydrogen obtained from the dehydrogenation of hydrocarbons and alcohols. In an alternative embodiment of this invention, the hydrogen may be generated in situ in the olefin oxidation process, for example, by dehydrogenating alkanes, such as propane or isobutane, or alcohols, such as isobutanol. Alternatively, hydrogen may be used to generate a catalyst-hydride complex or a catalyst-hydrogen complex which can provide the necessary hydrogen to the process.
Any quantity of hydrogen can be employed in the process provided that the amount is sufficient to produce the olefin oxide. Suitable quantities of hydrogen are typically greater than about 0.01, preferably, greater than about 0.1, and more preferably, greater than about 3 mole percent, based on the total moles of olefin, hydrogen, oxygen, and optional diluent. Suitable quantities of hydrogen are typically less than about 50, preferably, less than about 30, and more preferably, less than about 20 mole percent, based on the total moles of olefin, hydrogen, oxygen, and optional diluent.
In addition to the above reagents, it may be desirable to employ a diluent with the reactants, although the use thereof is optional. Since the process of this invention is exothermic, a diluent beneficially provides a means of removing and dissipating the heat produced. In addition the diluent provides an expanded concentration regime in which the reactants are non-flammable. The diluent can be any gas or liquid which does not inhibit the process of this invention. The specific diluent chosen will depend upon the manner in which the process is conducted. For example, if the process is conducted in a gas phase, then suitable gaseous diluents include, but are not limited to, helium, nitrogen, argon, methane, carbon dioxide, steam, and mixtures thereof. Most of these gases are essentially inert with respect to the process of this invention. Carbon dioxide and steam may not necessarily be inert, but may have a beneficial promoting effect. If the process is conducted in a liquid phase, then the diluent can be any oxidation stable and thermally stable liquid.
Examples of suitable liquid diluents include aliphatic alcohols, preferably C1-10 aliphatic alcohols, such as methanol and t-butanol; chlorinated aliphatic alcohols, preferably C1-10 chlorinated alkanols, such as chloropropanol; chlorinated aromatics, preferably chlorinated benzenes, such as chlorobenzene and dichlorobenzene; as well as liquid polyethers, polyesters, and polyalcohols.
If used, the amount of diluent is typically greater than about 0, preferably greater than about 0.1, and more preferably, greater than about 15 mole percent, based on the total moles of olefin, oxygen, hydrogen, and diluent. The amount of diluent is typically less than about 90, preferably, less than about 80, and more preferably, less than about 70 mole percent, based on the total moles of olefin, oxygen, hydrogen, and diluent.
The aforementioned concentrations of olefin, oxygen, hydrogen, and diluent are suitably based on the reactor designs and process parameters disclosed herein. Those skilled in the art will recognize that concentrations other than the aforementioned ones may be suitably employed in other various engineering realizations of the process.
The unique catalyst which is beneficially employed in the process of this invention comprises gold on a titanosilicate. Surprisingly, gold in combination with a titanosilicate can exhibit catalytic oxidation activity and enhanced selectivity for olefin oxides. Preferably, the catalyst of this invention is essentially free of palladium. The term xe2x80x9cessentially freexe2x80x9d means that the concentration of palladium is less than about 0.01 weight percent, preferably, less than about 0.005 weight percent, based on the total weight of the catalyst. More preferably, the catalyst of this invention is essentially free of the Group VIII metals, which means that the total concentration of these metals is less than about 0.01 weight percent, preferably, less than about 0.005 weight percent, based on the total weight of the catalyst. The Group VIII metals include iron, cobalt, nickel, ruthenium, rhodium, palladium, osmium, iridium, and platinum.
The gold predominantly exists as elemental metallic gold, as determined by X-ray photoelectron spectroscopy or X-ray absorption spectroscopy, although higher oxidation states may also be present. Most of the gold appears from TEM studies to be deposited on the surface of the titanosilicate; however, the deposition of individual gold atoms or small gold clusters in the pores or on any extra-framework titania or the inclusion of ionic gold into the silica framework may also occur. Preferably, the gold is not associated with any extra-framework titania or titania added as a support, as analyzed by TEM. Typically, the average gold particle size (or diameter) is about 10 xc3x85 or greater, as measured by TEM. Preferably, the average gold particle size is greater than about 10 xc3x85, more preferably, greater than about 12 xc3x85, and most preferably, greater than about 25 xc3x85. Preferably, the average gold particle size is less than about 500 xc3x85, more preferably, less than about 200 xc3x85, and most preferably, less than about 100 xc3x85.
The titanosilicate is characterized by a framework structure formed from SiO44xe2x88x92 tetrahedra and nominally TiO44xe2x88x92 tetrahedra. The titanosilicate can be crystalline, which implies that the framework has a periodic regularity which is identifiable by X-ray diffraction (XRD). Alternatively, the titanosilicate can be amorphous, which implies a random or non-periodic framework which does not exhibit a well-defined XRD pattern.
Any titanosilicate can be employed in the catalyst of this invention. Preferably, the titanosilicate is porous, which means that within the titanosilicate framework structure there exists a regular or irregular system of pores or channels. Empty cavities, referred to as xe2x80x9ccagesxe2x80x9d, can also be present. The pores can be isolated or interconnecting, and they can be one, two, or three dimensional. Preferably, the pores are micropores or mesopores or some combination thereof. For the purposes of this invention, a micropore has a pore diameter (or critical dimension as in the case of a non-circular perpendicular cross-section) ranging from about 4 xc3x85 to about 20 xc3x85, while a mesopore has a pore diameter or critical dimension ranging from greater than about 20 xc3x85 to about 200 xc3x85. The combined volume of the micropores and the mesopores preferably comprises about 70 percent or greater of the total pore volume, and preferably, about 80 percent or greater of the total pore volume. The balance of the pore volume comprises macropores which have a pore diameter of greater than about 200 xc3x85. These macropores will also include the void spaces between particles or crystallites.
The pore diameter (or critical dimension), pore size distribution, and surface area of the porous titanosilicate can be obtained from the measurement of adsorption isotherms and pore volume. Typically, the measurements are made on the titanosilicate in powder form using as an adsorbate nitrogen at 77 K or argon at 88 K and using any suitable adsorption analyzer, such as a Micromeritics ASAP 2000 instrument. Measurement of micropore volume is derived from the adsorption volume of pores having a diameter in the range from about 4 xc3x85 to about 20 xc3x85. Likewise, measurement of mesopore volume is derived from the adsorption volume of pores having a diameter in the range from greater than about 20 xc3x85 to about 200 xc3x85. From the shape of the adsorption isotherm, a qualitative identification of the type of porosity, for example, microporous or macroporous, can be made. Additionally, increased porosity can be correlated with increased surface area. Pore diameter (or critical dimension) can be calculated from the data using equations described by Charles N. Satterfield in Heterogeneous Catalysis in Practice, McGraw-Hill Book Company, New York, 1980, pp. 106-114, incorporated herein by reference.
Additionally, crystalline titanosilicates can be identified by X-ray diffraction(XRD), either by comparing the XRD pattern of the material of interest with a previously published standard or by analyzing the XRD pattern of a single crystal to determine framework structure, and if pores are present, the pore geometry and pore size.
Non-limiting examples of porous titanosilicates which are suitably employed in the process of this invention include porous amorphous titanosilicates; porous layered titanosilicates; crystalline microporous titanosilicates, such as titanium silicalite-1 (TS-1), titanium silicalite-2 (TS-2), titanosilicate beta (Ti-beta), titanosilicate ZSM-12 (Ti-ZSM-12) and titanosilicate ZSM-48 (Ti-ZSM-48); and mesoporous titanosilicates, such as Ti-MCM-41.
TS-1 possesses an MFI crystalline structure which is isomorphous to the crystalline structure of zeolite ZSM-5 and isomorphous to the structure of the pure silica form of ZSM-5 known as xe2x80x9csilicalite. The three-dimensional framework structure of the pure silica xe2x80x9csilicalitexe2x80x9d is formally constructed from tetrahedral SiO44xe2x88x92 units. In ZSM-5 some of the silica tetrahedra are replaced with AlO45xe2x88x92 tetrahedra, and a cation, such as sodium ion, is needed to balance charge requirements. In TS-1 some of the silica tetrahedra are replaced with TiO44xe2x88x92 tetrahedra. In this replacement, the overall charge remains electronically neutral and no additional cations are required. The pore structure of TS-1 comprises two interconnecting, roughly cylindrical, 10-ring pores of about 5 xc3x85 diameter. A 10-ring pore is formed from ten tetrahedral units. Titanium silicalite and its characteristic XRD pattern have been reported in U.S. Pat. No. 4,410,501, incorporated herein by reference. TS-1 can be obtained commercially, but it can also be synthesized following the methods described in U.S. Pat. No. 4,410,501. Other preparations have been reported by the following (incorporated herein by reference): A. Tuel, Zeolites, 1996, 16, 108-117; by S. Gontier and A. Tuel, Zeolites, 1996, 16, 184-195; by A. Tuel and Y. Ben Taarit in Zeolites, 1993, 13, 357-364; by A. Tuel, Y. Ben Taarit and C. Naccache in Zeolites, 1993, 13, 454-461; by A. Tuel and Y. Ben Taarit in Zeolites, 1994, 14, 272-281; and by A. Tuel and Y. Ben Taarit in Microporous Materials, 1993, 1, 179-189.
TS-2 possesses an MEL topology which is isomorphous to the topology of the aluminosilicate ZSM-11. The pore structure of TS-2 comprises one three-dimensional, microporous, 10-ring system. TS-2 can be synthesized by methods described in the following references (incorporated herein by reference): J. Sudhakar Reddy and R. Kumar, Zeolites, 1992, 12, 95-100; by J. Sudhakar Reddy and R. Kumar, Journal of Catalysis, 1991, 130, 440-446; and by A. Tuel and Y. Ben Taarit, Applied Catal. A, General, 1993, 102, 69-77.
Ti-beta possesses a BEA crystalline structure which is isomorphous to the aluminosilicate beta. The pore structure of Ti-beta comprises two interconnecting 12-ring, roughly cylindrical pores of about 7 xc3x85 diameter. The structure and preparation of titanosilicate beta have been described in the following references, incorporated herein by reference: PCT patent publication WO 94/02245 (1994); M. A. Camblor, A. Corma, and J. H. Perez-Pariente, Zeolites, 1993, 13, 82-87; and M. S. Rigutto, R. de Ruiter, J. P. M. Niederer, and H. van Bekkum, Stud. Surf. Sci. Cat., 1994, 84, 2245-2251.
Ti-ZSM-12 possesses an MTW crystalline structure which is isomorphous to the aluminosilicate ZSM-12. The pore structure of Ti-ZSM-12 comprises one, one-dimensional 12-ring channel system of dimensions 5.6xc3x977.7 xc3x85, as referenced by S. Gontier and A. Tuel, ibid., incorporated herein by reference.
Ti-ZSM-48 possesses a crystalline structure which is isomorphous to the aluminosilicate ZSM-48. The pore structure of Ti-ZSM-48 comprises a one-dimensional 10-ring channel system of dimensions 5.3 xc3x85 by 5.6 xc3x85, as referenced by R. Szostak, Handbook of Molecular Sieves, Chapman and Hall, New York, 1992, p. 551-553. Other references to the preparation and properties of Ti-ZSM-48 include C. B. Dartt, C. B. Khouw, H. X. Li, and M. E. Davis, Microporous Materials, 1994, 2, 425-437; and A. Tuel and Y. Ben Taarit, Zeolites, 1996, 15, 164-170, the aforementioned references being incorporated herein by reference.
Ti-MCM-41 is a hexagonal phase isomorphous to the aluminosilicate MCM-41. The channels in MCM-41 are one-dimensional with diameters ranging from about 28 xc3x85 to 100 xc3x85. Ti-MCM-41 can be prepared as described in the following citations, incorporated herein by reference: S. Gontier and A. Tuel, Zeolites, 1996, 15, 601-610; and M. D. Alba, Z. Luan, and J. Klinowski, J. Phys. Chem., 1996, 100, 2178-2182.
The silicon to titanium atomic ratio (Si/Ti) of the titanosilicate can be any ratio which provides for an active and selective epoxidation catalyst in the process of this invention. A generally advantageous Si/Ti atomic ratio is equal to or greater than about 5/1, and preferably, equal to or greater than about 10/1. A generally advantageous Si/Ti atomic ratio is equal to or less than about 200/1, preferably, equal to or less than about 100/1. The Si/Ti atomic ratio defined hereinabove refers to a bulk ratio which includes the total of the framework titanium and the extra-framework titanium. At high Si/Ti ratios, for example, about 100/1 or more, there may be little extra-framework titanium and the bulk ratio essentially corresponds to the framework ratio.
In one preferred embodiment of this invention, the catalyst is substantially free of the anatase phase of titanium dioxide, more preferably, substantially free of crystalline titanium dioxide, and most preferably, free of titanium dioxide. Crystalline titanium dioxide may be present, for example, as extra-framework titania or titania added as a carrier or support. Raman spectroscopy can be used to determine the presence of crystalline titanium dioxide. The anatase phase of titanium dioxide exhibits a characteristic strong, sharp Raman peak at about 147 cmxe2x88x921. The rutile phase exhibits Raman peaks at about 448 cmxe2x88x921 and about 612 cmxe2x88x921. The brookite phase, which usually is available only as a natural mineral, exhibits a characteristic peak at about 155 cmxe2x88x921. The rutile and brookite peaks have a lower intensity than the 147 cmxe2x88x921 peak of anatase. In the aforementioned more preferred embodiment of the catalyst, Raman peaks for the anatase, rutile, and brookite phases of titanium dioxide are essentially absent. When the catalyst exhibits essentially no detectable peaks at the aforementioned wavenumbers, then it is estimated that less than about 0.02 weight percent of the catalyst exists in the form of crystalline titanium dioxide. Raman spectra can be obtained on any suitable laser Raman spectrometer equipped, for example, with an argon ion laser tuned to the 514.5 nm line and having a laser power of about 90 to 100 mW measured at the sample.
The loading of the gold on the titanosilicate can be any loading which gives rise to the desired olefin oxide product. Generally, the gold loading is greater than about 0.01 weight percent, based on the total weight of gold and titanosilicate. Generally, the loading is less than about 20 weight percent. Preferably, the gold loading is greater than about 0.03, more preferably, greater than about 0.05 weight percent. Preferably, the gold loading is less than about 10.0, more preferably, less than about 5.0 weight percent.
The gold component can be deposited or supported on the titanosilicate by any method known in the art which provides for an active and selective catalyst. Non-limiting examples of known deposition methods include impregnation, ion-exchange, and deposition by precipitation. A preferred deposition method is disclosed by S. Tsubota, M. Haruta, T. Kobayashi, A. Ueda, and Y. Nakahara, xe2x80x9cPreparation of Highly Dispersed Gold on Titanium and Magnesium Oxide,xe2x80x9d in Preparation of Catalysts V, G. Poncelet, P. A. Jacobs, P. Grange, and B. Delmon, eds., Elsevier Science Publishers B. V., Amsterdam, 1991, p. 695ff, incorporated herein by reference. This method involves contacting the titanosilicate with an aqueous solution of a soluble gold compound at a temperature and pH sufficient to precipitate the gold compound onto the titanosilicate. Non-aqueous solutions can also be employed. Thereafter, in the preferred method of this invention which is different from the aforementioned reference, the gold/titanosilicate composite is not washed or is lightly washed, with preferably no more than about 100 ml wash liquid per gram composite. Then, the composite is calcined or reduced at a temperature sufficient to reduce the gold substantially to metallic gold having an average particle size between about 10 xc3x85 and about 500 xc3x85.
For aqueous solvents, any water soluble gold compound can be used, such as chloroauric acid, sodium chloroaurate, potassium chloroaurate, gold cyanide, potassium gold cyanide, and diethylamine auric acid trichloride. Typically, the molarity of the soluble gold compound ranges from about 0.001 M to the saturation point of the soluble gold compound, preferably, from about 0.005 M to about 0.5 M. The desired quantity of titanosilicate is added to the solution, or vice versa; and the pH is adjusted to between about 5 and about 11, preferably, between about 6 and about 9, with any suitable base, such as a Group 1 hydroxide or carbonate, preferably, sodium hydroxide, sodium carbonate, potassium carbonate, cesium hydroxide, and cesium carbonate. Thereafter, the mixture is stirred under air at a temperature between about 20xc2x0 C. and about 80xc2x0 C. for a time ranging from about 1 hour to about 15 hours. At the end of this period, the solids are recovered and optionally washed with water, the water optionally containing promoter metal salts, described hereinbelow, preferably at a pH between about 5 and 11. Typically thereafter, the solids are dried under air at a temperature between about 80xc2x0 C. and about 110xc2x0 C. The solid is then calcined under air, or calcined in a reducing atmosphere, such as hydrogen, or heated in an inert atmosphere, such as nitrogen, at a temperature between about 250xc2x0 C. and about 800xc2x0 C. for a time from about 1 hour to about 24 hours to form a titanosilicate having metallic gold thereon.
Optionally, the catalyst of this invention can contain a promoter metal or a combination of promoter metals. Any metal ion having a valence between +1 and +7 which enhances the productivity of the catalyst in the oxidation process of this invention can be employed as a promoter metal. Factors contributing to increased productivity of the catalyst include increased conversion of the olefin, increased selectivity to the olefin oxide, decreased production of water, and increased catalyst lifetime. Non-limiting examples of suitable promoter metal include the metals of Groups 1 through 12 of the Periodic Table of the Elements, as well as the rare earth lanthanides and actinides, as referenced in the CRC Handbook of Chemistry and Physics, 75th ed., CRC Press, 1994. Preferably, the promoter metal is selected from Group 1 metals of the Periodic Table including lithium, sodium, potassium, rubidium, and cesium; from Group 2 metals, including beryllium, magnesium, calcium, strontium, and barium; from the lanthanide rare earth metals, including cerium, praseodymium, neodymium, promethium, samarium, europium, gadolinium, terbium, dysprosium, holmium, erbium, thulium, ytterbium, and lutetium; and the actinide metals, specifically, thorium and uranium, or from a combination of these metals. More preferably, the promoter metal is magnesium, calcium, barium, erbium, lutetium, lithium, potassium, rubidium, cesium, or a combination thereof. In another preferred embodiment, the promoter metal excludes palladium, and even more preferably, excludes a Group VIII metal, including, iron, cobalt, nickel, ruthenium, rhodium, palladium, osmiurn, iridium, and platinum. As used herein the word xe2x80x9cexcludesxe2x80x9d means that the concentration of the metal is less than about 0.01, preferably, less than about 0.005 weight percent, based on the total weight of the catalyst.
If one or more promoter metals arc used as described hereinabove, then the total quantity of promoter metal(s) generally is greater than about 0.01, preferably, greater than about 0.10, and more preferably, greater than about 0.15 weight percent, based on the total weight of the catalyst. The total quantity of promoter metal(s) is generally less than about 20, preferably, less than about 15, and more preferably, less than about 10 weight percent, based on the total weight of the catalyst.
The promoter metal(s) can be deposited or supported onto the titanosilicate simultaneously with the gold particles, or alternatively, in a separate step either before or after the gold is deposited or supported. Generally, the promoter metal is deposited from an aqueous or organic solution containing a soluble promoter metal salt. Any salt of the promoter metal which has adequate solubility can be used; for example, the metal nitrates, carboxylates, and halides, preferably the nitrates, are suitable. If an organic solvent is employed, it can be any of a variety of known organic solvents, including, for example, alcohols, esters, ketones, and aliphatic and aromatic hydrocarbons. Ordinarily, the titanosilicate is contacted with the solution of the promoter metal salt under conditions which are similar to those used for contacting the titanosilicate with the gold solution. After the promoter metal is deposited, washing is optional. If done to excess, washing can leach at least a portion of the promoter metal out of the catalyst. Afterwards, calcination under air or under a reducing atmosphere or heating in an inert gas is conducted in a manner similar to that described hereinabove for the gold deposition.
Optionally, the catalyst of this invention can be extruded with, bound to, or supported on a second support, such as silica, alumina, an aluminosilicate, magnesia, titania, carbon, or mixtures thereof. The second support may function to improve the physical properties of the catalyst, such as, the strength or attrition resistance, or to bind the catalyst particles together. Generally, the quantity of second support ranges from about 0 to about 95 weight percent, based on the combined weight of the catalyst and second support. It is noted that although the catalyst of this invention can be physically mixed or extruded with titania or bound to titania as a second support, in a preferred embodiment the catalyst is substantially free of the anatase phase of titanium dioxide, more preferably free of crystalline titanium dioxide, as noted hereinabove. If titania is used as a second support, however, note that its presence may interfere with the analytical identification of the catalyst. In this instance especially, analysis of the catalyst should be made in the absence of the second support.
The process of this invention can be conducted in a reactor of any conventional design suitable for gas or liquid phase processes. These designs broadly include batch, fixed-bed, transport bed, fluidized bed, moving bed, trickle bed, and shell and tube reactors, as well as continuous and intermittent flow and swing reactor designs. Alternatively, the process may be conducted in two-steps wherein the catalyst is first contacted with oxygen and thereafter the oxygenated catalyst is contacted with a mixture of propylene and hydrogen. Preferably, the process is conducted in the gas phase and the reactor is designed with heat transfer features for the removal of the heat produced. Preferred reactors designed for these purposes include fixed-bed, shell and tube, fluidized bed, and moving bed reactors, as well as swing reactors constructed from a plurality of catalyst beds connected in parallel and used in an alternating fashion.
The process conditions for the direct oxidation described herein can vary considerably over a nonflammable and flammable regime. It is beneficial, however, to recognize the conditions which distinguish between nonflammable and flammable mixtures of the olefin, hydrogen, and oxygen. Accordingly, a phase diagram can be constructed or consulted which for any given process temperature and pressure shows the flammable and non-flammable range of reactant compositions, including the diluent, if used. The more preferred reactant mixtures specified hereinabove arc believed to lie outside the flammable regime when the process is operated at the more preferred temperatures and pressures specified hereinbelow. Nevertheless, operation within the flammable regime is possible, as designed by one skilled in the art.
Usually, the process is conducted at a temperature which is greater than about ambient, taken as 20xc2x0 C., preferably, greater than about 70xc2x0 C., more preferably greater than about 120xc2x0 C. Usually, the process is conducted at a temperature less than about 250xc2x0 C., preferably less than about 225xc2x0 C., more preferably, less than about 200xc2x0 C. Preferably, the pressure ranges from about atmospheric to about 400 psig (2758 kPa), more preferably, from about 150 psig (1034 kPa) to about 250 psig (1724 kPa).
In flow reactors the residence time of the reactants and the molar ratio of reactants to catalyst will be determined by the space velocity. For a gas phase process the gas hourly space velocity (GHSV) of the olefin can vary over a wide range, but typically is greater than about 10 ml olefin per ml catalyst per hour (hrxe2x88x921), preferably greater than about 100 hrxe2x88x921, and more preferably, greater than about 1,000 hrxe2x88x921. Typically, the GHSV of the olefin is less than about 50,000 hrxe2x88x921, preferably, less than about 35,000 hrxe2x88x921, and more preferably, less than about 20,000 hrxe2x88x921. Likewise, for a liquid phase process the weight hourly space velocity (WHSV) of the olefin component may vary over a wide range, but typically is greater than about 0.01 g olefin per g catalyst per hour (hrxe2x88x921), preferably, greater than about 0.05 hrxe2x88x921, and more preferably, greater than about 0.1 hrxe2x88x921. Typically, the WHSV of the olefin is less than about 100 hrxe2x88x921, preferably, less than about 50 hrxe2x88x921, and more preferably, less than about 20 hrxe2x88x921. The gas and weight hourly space velocities of the oxygen, hydrogen, and diluent components can be determined from the space velocity of the olefin taking into account the relative molar ratios desired.
When an olefin having at least three carbon atoms is contacted with oxygen in the presence of hydrogen and the catalyst described herein-above, the corresponding olefin oxide (epoxide) is produced in good productivity. The most preferred olefin oxide produced is propylene oxide.
The conversion of olefin in the process of this invention can vary depending upon the specific process conditions employed, including the specific olefin, temperature, pressure, mole ratios, and form of the catalyst. As used herein, the term xe2x80x9cconversionxe2x80x9d is defined as the mole percentage of olefin which reacts to form products. Generally, the conversion increases with increasing temperature and pressure and decreases with increasing gas hourly space velocity. Typically, an olefin conversion of greater than about 0.05 mole percent is achieved. Preferably, the olefin conversion is greater than about 0.2 percent.
Likewise, the selectivity to olefin oxide can vary depending upon the specific process conditions employed. As used herein, the term xe2x80x9cselectivityxe2x80x9d is defined as the mole percentage of reacted olefin which forms a particular product, desirably the olefin oxide. Generally, the selectivity to olefin oxide will decrease with increasing temperature and increase with increasing space velocity. The process of this invention produces olefin oxides in unexpectedly high selectivity. A typical selectivity to olefin oxide in this process is greater than about 50, preferably, greater than about 70, and more preferably, greater than about 90 mole percent. A selectivity to propylene oxide of greater than about 99 mole percent at 50xc2x0 C. has been achieved. Even at 165xc2x0 C. the selectivity to propylene oxide is surprisingly high, between about 85 and 95 mole percent.
Advantageously, the hydrogen efficiency in the process of this invention is satisfactory. Some additional hydrogen may be burned directly to form water. Accordingly, it is desirable to achieve a water/olefin oxide molar ratio as low as possible. In the process of this invention, the water/olefin oxide molar ratio is typically greater than about 2/1, but less than about 15/1, and preferably, less than about 10/1, and more preferably, less than about 7/1.
The catalyst of this invention exhibits evidence of a long lifetime. The term xe2x80x9clifetimexe2x80x9d as used herein refers to the time measured from the start of the oxidation process to the point at which the catalyst after regeneration has lost sufficient activity so as to render the catalyst useless, particularly commercially useless. As evidence of its long lifetime, the catalyst remains active for long periods of time with little deactivation. Typically, a run time greater than about 100 hours without catalyst deactivation has been achieved in a fixed bed reactor. In a preferred mode, a run time greater than about 550 hours without catalyst deactivation has been achieved. The preferred run time between regenerations will depend upon the reactor design and may range from minutes for transport bed reactors to several months for fixed bed reactors. As further evidence of its longevity, the catalyst of this invention can be regenerated through multiple cycles without substantial loss in catalyst activity or selectivity.
When its activity has decreased to an unacceptably low level, the catalyst of this invention can be easily regenerated. Any catalyst regeneration method generally known to those skilled in the art can be used with the catalyst of this invention provided that the catalyst is reactivated for the oxidation process described herein. One suitable regeneration method comprises heating the deactivated catalyst at a temperature between about 150xc2x0 C. and about 500xc2x0 C. under an atmosphere of a regeneration gas containing hydrogen and/or oxygen and optionally an inert gas. A preferred regeneration temperature varies between about 200xc2x0 C. and about 400xc2x0 C. The amounts of hydrogen and/or oxygen in the regeneration gas can be any which effectively regenerates the catalyst. Preferably, the hydrogen and/or oxygen comprises from about 2 to about 100 mole percent of the regeneration gas. Suitable inert gases are non-reactive and include, for example, nitrogen, helium, and argon. The regeneration cycle time, that is the time during which the catalyst is being regenerated, can range from as little as about 2 minutes to as long as several hours, for example, about 20 hours at the lower regeneration temperatures. In an alternative embodiment, water is beneficially added to the regeneration gas in an amount preferably ranging from about 0.01 to about 100 mole percent.
The invention will be further clarified by a consideration of the following examples, which are intended to be purely exemplary of the use of the invention. Other embodiments of the invention will be apparent to those skilled in the art from a consideration of this specification or practice of the invention as disclosed herein. Unless otherwise noted, all percentages arc given on a weight percent basis.
Preparation of Titanium Silicalite TS-1 having Si/Ti=100
Tetraethylorthosilicate (Fisher TEOS, 832.5 g) was weighed into a 4 liter stainless steel beaker and sparged with nitrogen gas for 30 minutes. Titanium n-butoxide (DuPont, Ti(O-n-Bu)4) was injected from a syringe into the silicate. The weight of the titanium n-butoxide which was added to the TEOS was 14.07 g, taken by difference. A clear yellow solution was formed. The solution was heated and stirred under nitrogen for about 3 hr. The temperature varied from 50xc2x0 C. to 130xc2x0 C. The solution was then chilled in an ice bath.
A 40 percent aqueous solution of tetrapropylammonium hydroxide (TPAOH, 710.75 g) was weighed into a polyethylene bottle, which was capped and placed in an ice bath. The TPAOH was added dropwise to the chilled TEOS solution with vigorous stirring by an overhead stirrer. After one-half of the TPAOH had been added, the TEOS solution was cloudy and began to thicken. Within five minutes the solution froze completely. At this point the remainder of the TPAOH was added, the gel was broken up with a spatula, and stirring was resumed. Deionized water (354 g) was added, and the solution was warmed to room temperature. After 5 hr the solids had largely dissolved, and an additional quantity of deionized water (708 g) was added. Stirring was continued overnight yielding a clear yellow synthesis gel containing no solids.
The synthesis gel was poured into a 1 gallon (3.785 liters) stainless steel autoclave and sealed. The autoclave was heated to 120xc2x0 C. and then gradually to 160xc2x0 C. where it was kept for 6 days. The reactor contents were stirred at all times. At the end of the reaction period, the autoclave was cooled and a milky white suspension was recovered. The solids were recovered, washed, centrifuged, and resuspended in deionized water. The solids were filtered, dried at room temperature, heated slowly to 550xc2x0 C., and calcined thereat for 8 hr. The solid was identified as having an MFI structure, as determined by XRD. Raman spectra did not reveal any known crystalline titania phase. A Si/Ti atomic ratio of 100 was found, as measured by X-ray fluorescence (XRF). Yield of titanium silicalite-1: 106 g.